Group III nitride semiconductors are widely used for efficient blue and ultraviolet light emitting diodes, lasers, ultraviolet detectors, and field effect transistors. Due to a wide band-gap, group III nitride semiconductor materials are one of the prime choices for deep ultraviolet light emitting diodes (DUV LEDs). While there has been much advancement in improving the efficiency of DUV LEDs, the overall efficiency of these devices remains low. The wide band-gap of group III nitride semiconductor materials makes it difficult to form a good ohmic contact to the semiconductor layers. This can lead to resistive losses at the contact junction.
DUV LED devices frequently employ flip-chip technology in order to control light extraction and thermal management of DUV LED devices. For example, FIG. 1 illustrates a typical design of a flip-chip LED 2 according to the prior art. For flip-chip LEDs to have a high efficiency, it is desirable for the p-type contact 6 and the n-type contact 8 to be both ohmic and reflective, which allows each contact 6, 8 to serve as an electrode as well as a mirror for reflecting light emitted by an active region 4 out of the device 2. Additionally, it is desirable for the contacts 6, 8 to have stability during thermal cycling that occurs while packaging, as well as during operation of the device. Aluminum is a good reflecting metal, however, aluminum does not produce ohmic contact and is unstable during packaging.
Several types of metallic contacts have been proposed to improve ohmic contact to semiconductor layers. These contacts are formed of, for example: nickel/gold (Ni/Au), cobalt/gold (Co/Au), palladium/gold (Pd/Au), rhodium (Rh)-based, palladium/platinum/gold (Pd/Pt/Au), Pt/Ni/Au, Ni/Pt/Au, Ni/Pd/Au, and titanium/platinum/gold (Ti/Pt/Au) metallic layers. The thermal stability of Pd/Ni contacts is enhanced due to the formation of Pd gallides. Additionally, Pd/Ni contacts can lead to a reduction of contact resistivity. For Ni-based contacts, the Ni is easily oxidized above 400 degrees Celsius and the ohmic contact becomes worse at temperatures above 500 degrees Celsius.
One approach proposed a Pd/Ni/Al/Ti/Au metallization scheme for a contact with layers having corresponding thicknesses of 3 nm/2 nm/150 nm/20 nm/30 nm. This contact exhibited good thermal stability and reflectivity of sixty-two percent for radiation having a wavelength of 370 nm at normal incidence and good ohmic characteristics after annealing at three hundred degrees Celsius in nitrogen gas (N2). It is further noted that the combination of Pd and Ni results in a good ohmic contact, while a contact without the presence of Ni results in larger resistance and non-linear behavior.
Most attempts at contact engineering have been for visible LEDs or near ultraviolet (UV) LEDs. For example, one approach found good ohmic properties for an iridium/silver (Ir/Ag) p-type contact with a seventy-five percent reflectivity for radiation with a wavelength of 405 nm. At the same wavelength, an indium-doped zinc oxide/silver (ZnO/Ag) contact has a reflectivity of 82.3%.
Other attempts at contact engineering for radiation near UV wavelengths have been proposed. For example, one approach proposed a nickel (Ni) “cleaning” mechanism. Residual oxide on the Gallium Nitride (GaN) surface was removed by Ni deposition and subsequent annealing. This resulted in better ohmic contact properties. Another approach proposed indium tin oxide (ITO) and zinc oxide (ZnO) contacts, instead of metallic contacts. However, for UV LEDs, different contacts are required in order to result in highly reflective UV mirrors.
The current application incorporates by reference U.S. Provisional Application No. 61/569,416 titled “Ultraviolet Reflective Contact,” which was filed on 12 Dec. 2011. This provisional application outlines a contact that comprises at least two or more original sublayers which may comprise an ohmic sublayer, ohmic protection sublayer, reflective sublayer, reflector protective sublayer, conductive electrode sublayer and a final layer being dielectric adhesion layer. In addition to a metallic sublayer structure of the contact, the p-type group III semiconductor material also may contain a sublayered structure that may contain a thin layer of p-type GaN in the vicinity of the p-type contact together with a graded GaN—AlGaN region. Furthermore, a thin layer of InxAlyGa1-x-yN layer may be included in the vicinity of the p-type contact. The provisional application also considers the possibility of a contact having an inhomogeneous structure in a lateral direction, both in the metallic layers as well as composition in the underlying group III semiconductor layers.